[headcrab13]

How about this:

Once a week, a nuke is assembled at a single base (only one base on the whole planet). This would really heat up the fight in the hours prior to nuke creation. All three empires would be gunning for that base and the surrounding region.

After the long fight for nuke control, the victorious empire would have to transport the nuke by ground convoy to a base on the other side of the continent. The other two empires would be free to plan ambushes and chase attacks on the moving convoy, and destroying the nuke vehicle before the weapon was armed would neutralize the threat.

Once secured in the second base, the nuke could be armed and launched after a 15-minute countdown. The opposition can break into the launch control room at any time and hack it back, if they're able to clear a path to the terminal and secure it.

If the other two empires fail to stop the nuke's arming sequence, the controlling empire can fire it at any point on the planet, killing everything in a massive area and allowing them to move in and take all the bases in that area without a fight.